This invention relates to hydraulic systems having a positive displacement pump for supplying pressurized fluid to hydraulically-operated devices, and more particularly to electro-hydraulic mechanisms for controlling the output flow of the pump.
Positive displacement pumps are used to supply pressurized fluid in automatic transmissions for providing power to actuate various torque transmitting elements such as clutches, brakes and torque converters. The pump is generally driven directly by the engine at the transmission input or through a gear train at the transmission input. The pump is a positive displacement type pump of the gear type or vane type. The vane type pumps can be a variable displacement device.
The output pressure of the pump is established by a pressure regulator valve that is controlled by a control system which may be either hydraulic or electro-hydraulic in nature. Most modern automatic transmissions have electro-hydraulic controls which provide a more versatile control system in both pressure control and power transmission control.
Since the pumps are driven at engine speed, the maximum output volume at any particular instant is determined by the engine speed at that time. Also, the pump is driven at high speeds when the vehicle is travelling at highway speeds. With a fixed displacement pump, the volume output is very high at the high engine speeds which can result in cavitation at the pump inlet which results in an undesirable noise situation. This condition also results in a decrease in system efficiency since much of the pump volume is not needed and is therefor returned to the transmission sump of the system cooler.
While variable displacement type pumps alleviate some of the problems, they do not address all of them. During engine idle or slow speed at high operating temperatures, the pump output can be significantly diminished due to leakage in the pump structure. Under these circumstances, the output pressure can be lower than desirable. This can introduce slower than desired ratio interchanges and low apply pressures at the friction devices in the transmission. Also the manufacturing cost of the variable displacement pump and related controls is often higher than the cost of a fixed displacement pump.
It is an object of the present invention to provide an improved control for a transmission supply pump.
In one aspect of the present invention, a positive displacement pump is driven by an electric motor which is speed controlled to provide an output pressure between defined limits. In another aspect of the present invention, the output pressure is controlled by a regulator valve which receives signals from a transmission electro-hydraulic control.
In yet another aspect of the present invention, the low pressure limit of the regulator valve is sensed by a pressure activated switch that provides a signal to the transmission control requesting a pump speed increase to increase the pump output volume. In still another aspect of the present invention, the high pressure limit is sensed by a switch that signals the transmission control to reduce the speed of the pump to reduce the pump output flow.
The control system includes an electric motor driven pump for supplying hydraulic fluid to an electro-hydraulic mechanism for controlling various transmission functions such as ratio interchanges, the apply pressure for various friction devices, fluid feed to the torque converter and clutch and various control pressure signals. The pump output pressure is established by a multi-stage regulator valve that supplies main system pressure fluid to the electro-hydraulic control portion of the transmission. The electro-hydraulic control establishes many control pressure levels through the use of variable pressure devices such as pwm solenoids and variable bleed solenoids. These pressure levels are used to provide ratio interchange signals, speed signals and throttle or torque demand signals to name a few.
When the main system pressure level is satisfied, the regulator valve distributes pressurized fluid to the torque converter and clutch. After this subsystem is satisfied, the regulator valve distributes fluid to a lubrication and cooling circuit and also returns excess fluid to the transmission reservoir. Two pressure activated devices are included in the present invention to accommodate a low pressure limit and a high pressure limit. If either of these pressure activated devices changes state, the central processing unit (CPU) in the electro-hydraulic control is effective to change motor speed at the pump in accordance with a stored array of data such that the pump output will be adjusted accordingly.